1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the invention relates to a device which is capable of removing NO.sub.X components in the exhaust gas from a lean burn engine at a high efficiency.
2. Description of the Related Art
An exhaust gas purification device utilizing a three-way reducing and oxidizing catalyst (hereinafter referred to as a "three-way catalyst") is commonly used for removing HC, CO and NO.sub.X components from the exhaust gas of an internal combustion engine (in this specification, the term NO.sub.X means a nitrogen oxide such as NO, NO.sub.2, N.sub.2 O and N.sub.2 O.sub.4 in general). The three-way catalyst is capable of oxidizing HC and CO components, and reducing NO.sub.X components in the exhaust gas, when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio. Namely, the three-way catalyst is capable of removing these harmful components from exhaust gas simultaneously when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio.
However, the ability of the three-way catalyst for reducing NO.sub.X components becomes lower as the air-fuel ratio of the exhaust gas becomes lean (i.e., as the air-fuel ratio becomes higher than the stoichiometric air-fuel ratio). Therefore, it is difficult to remove NO.sub.X components in the exhaust gas from a lean burn engine which is operated at, as a whole, a lean air-fuel ratio using a three-way catalyst.
To solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 4-365920 discloses an exhaust gas purification device utilizing a denitrating reaction.
When the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio (i.e., when the air-fuel ratio of the exhaust gas is rich), the three-way catalyst converts a portion of NO.sub.X in the exhaust gas to NH.sub.3 while reducing most of NO.sub.X in the exhaust gas and converting it into N.sub.2. The device in the '920 publication produces NH.sub.3 from NO.sub.X in the exhaust gas using a three-way catalyst, and reacts the produced NH.sub.3 with the NO.sub.X components in the exhaust gas to reduce NO.sub.X to N.sub.2 and H.sub.2 O by a denitrating reactions.
In the '920 publication, a multiple-cylinder internal combustion engine is used, and a group of cylinders of the engine are operated at a rich air-fuel ratio while other cylinders are operated at a lean air-fuel ratio. Further, a three-way catalyst having a high capability for converting NO.sub.X to NH.sub.3 is disposed in an exhaust gas passage connected to the rich air-fuel ratio cylinders. After it flows through the three-way catalyst, the exhaust gas from the rich air-fuel ratio cylinders mixes with the exhaust gas from the lean air-fuel ratio cylinders. Since, when the exhaust gas from the rich air-fuel ratio cylinders flows through the three-way catalyst, a portion of NO.sub.X components in the exhaust gas is converted to an NH.sub.3 component, the exhaust gas downstream of the three-way catalyst contains a relatively large amount of NH.sub.3. On the other hand, the exhaust gas from the lean air-fuel ratio cylinders contains a relatively large amount of NO.sub.X. Therefore, by mixing the exhaust gas from the three-way catalyst and the exhaust gas from the lean air-fuel ratio cylinders, NH.sub.3 in the exhaust gas from the three-way catalyst reacts with the NO.sub.X components in the exhaust gas from the lean air-fuel ratio cylinder, and NH.sub.3 and NO.sub.X components produce N.sub.2 and H.sub.2 O by a denitrating reaction. Thus, according to the device in the '920 publication, the NO.sub.X components are removed from the exhaust gas.
In the device in the '920 publication, it is required that the amount of NH.sub.3 produced by the three-way catalyst is sufficient for reducing all the amount of NO.sub.X in the exhaust gas from the lean air-fuel ratio cylinders. For example, the greatest part of NO.sub.X in the exhaust gas discharged from the engine is composed of NO (nitrogen monoxide) and NO.sub.2 (nitrogen dioxide) components. These NO and NO.sub.2 components react with NH.sub.3 and produce N.sub.2 and H.sub.2 O by the following denitrating reaction. EQU 4NH.sub.3 +4NO+O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O EQU 8NH.sub.3 +6NO.sub.2 .fwdarw.7N.sub.2 +12H.sub.2 O
Therefore, in the device in the '920 publication, an amount of NH.sub.3 which equals to a total of the number of moles of NO and 4/3 times the number of moles of NO.sub.2 is required to remove all of NO.sub.X component in the exhaust gas from the lean air-fuel ratio cylinders. When the exhaust gas contains other NO.sub.X components such as N.sub.2 O.sub.4, N.sub.2 O components, the amount of NH.sub.3 stoichiometrical to the amount of these components is required in addition to the above noted amount on NH.sub.3.
However, the amount of NO.sub.X produced in the cylinders of the engine becomes the maximum when the cylinders are operated at a lean air-fuel ratio (for example, at an air-fuel ratio about 16), and decreases rapidly when the cylinders are operated at a rich air-fuel ratio. Since the device in the '920 publication converts NO.sub.X in the exhaust gas of the rich air-fuel ratio cylinder to produce NH.sub.3, the amount of NH.sub.3 obtained is limited by the amount of NO.sub.X produced in the rich air-fuel ratio cylinders. Therefore, in the device of the '920 publication, the amount of NH.sub.3 produced by the three-way catalyst is not sufficient to reduce all the amount of NO.sub.X in the exhaust gas from the lean air-fuel ratio cylinders, and a part of NO.sub.X in the exhaust gas from the lean air-fuel ratio cylinder is released to the atmosphere without being reduced.